1. Field of the Invention
The present invention relates to a transmitter for differential signaling. More specifically, a transmitter for differential signaling outputting selectable low-voltage currents is disclosed.
2. Description of the Prior Art
Swing differential signaling is a low-voltage technology used in data transmission systems. The use of low-voltage differential signaling for data transmission has grown rapidly due to the low power dissipation, high signal-to-noise ratio, low EMI emission, and high transmission speed characteristics inherent in such a system. Today's differential signaling systems usually have a swing, or peak-to-peak amplitude of 600 mv or less, depending on the particular derivation in use.
Please refer to FIG. 1 that is a simple circuit diagram of a prior art differential signaling transmitter 10. Transmitter 10 comprises a bias 12 regulating two electrical sources 14, 16, a ground 32, two transistors 24, 26, two inverted transistors 20, 22, and a resistor 18 (R2). Transmitter 10 also comprises two inputs, a first input for data and a second input for inverted data (data bar).
The bias 12 controls the inputted current of the current source 14 to fall within a specified range. When the inputted data is high (thus data bar is low) transistors 24 and 22 are turned on allowing current to flow from the current source 14 through transistor 22 to a node NB. From the Node NB, the current VoutP flows to the resistor 18. From the resistor 18, the current VoutN flows to a node NA and through the transistor 24 and the current source 16 to the ground 32. When the inputted data is low (thus data bar is high) transistors 20 and 26 are turned on allowing current to flow from the current source 14 through transistor 20 to the node NA. From the node NA, the current VoutN flows to the resistor 18 (R2). From the resistor 18, the current VoutP flows to the ground 32 via the node NB, the transistor 26, and the current source 16. The outputted voltage when data is high is equal to VoutP−VoutN which equals I*R2 and corresponds to a “1” in the differential signal. The outputted voltage when data is low is equal to VoutP−VoutN which equals −I*R2 and corresponds to a “0” in the differential signal.
Currently there are at least three major types of swing differential signaling systems commonly used. First, is a Low Voltage Differential Signaling (LVDS) system with signal swings in the 247 mv-454 mv range. A swing of 350 mv would be considered typical for LVDS. Next is a Mini-LVDS system with signal swings in the 300 mv-600 mv range, typically about 450 mv. Thirdly, a Reduced Swing Differential Signaling (RSDS™) system generally supports a signal swing of 200 mv but includes a signal swing range of 100 mv-400 mv.
The benefits provided by a low-voltage differential signaling transmitter 10 can be offset when different applications within the same system require different swing voltage levels. The choice of which kind of differential signaling transmitter 10 to use is related to numerous design considerations including bandwidth required, length of the connection, driver transition time, and signal quality, all of which are application specific and often incompatible with each other. In such a scenario, system designers or administrators are faced with a choice of decreased efficiency or the cost and complications additional transmitters 10 for each voltage range needed.